The invention relates to a process for the preparation of polycarbonates, the polycarbonate substrates of particularly high purity obtainable by the process and the shaped articles which can be produced from the polycarbonate substrate.
Polycarbonate is prepared by the so-called phase boundary process, in which dihydroxydiarylalkanes in the form of their alkali metal salts are reacted with phosgene in the heterogeneous phase in the presence of inorganic bases, such as sodium hydroxide solution, and an organic solvent in which the product, polycarbonate, is readily soluble. During the reaction the aqueous phase is dispersed in the organic phase, and after the reaction the organic polycarbonate-containing phase is washed with an aqueous liquid, during which, inter alia, electrolytes are to be removed, and the wash liquid is then separated off (H. Schnell xe2x80x9cChemistry and Physics of Polycarbonatesxe2x80x9d, Polymer Review, vol. IX p. 33 et seq, Interscience Publishers, New York 1964).
For washing the polycarbonate-containing solution, EP-A-264885 proposes stirring the aqueous wash liquid with the polycarbonate solution and separating out the aqueous phase by centrifuging.
Japanese Application JP-A-07 19 67 83 describes a process for the preparation of polycarbonate in which the content of iron in the sodium hydroxide solution used should be below 2 ppm in order to achieve favourable colour properties.
The object of the present invention is to provide an alternative and improved process for the preparation of pure polycarbonate or polycarbonate substrates and to provide polycarbonate shaped articles with a low foreign particle content.
Surprisingly, it has now been found that polycarbonate and polycarbonate substrates with a low foreign particle content are obtained if a specific process is used.
The present application therefore provides a process for the preparation of polycarbonate by the phase boundary process, in which dihydroxydiarylalkanes in the form of their alkali metal salts are reacted with phosgene in the heterogeneous phase in the presence of sodium hydroxide solution and an organic solvent, characterised in that
a) the feedstocks are low in Fe, Cr, Ni, Zn, Ca, Mg and Al metals or their homologues;
b) the organic solvent is separated off and
c) the polycarbonate obtained is worked up.